1. Field of the Invention
The present invention relates generally to a process for vacuum degassing and decarbonization of molten steel, such as closed circuit vacuum degassing process Ruhrstahl Hausen (RH) process, RH-OB process and so forth. More specifically, the invention relates to a technology of temperature compensation in vacuum degassing and decarbonization process.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 52-5614 discloses RH process for performing decarbonization under vacuum pressure. On the other hand, Japanese Patent First Publication (Tokkai) Showa 51-140815 discloses a technology of injecting oxygen gas into a molten steel bath through a side wall of a ladle. The later technology is explained to be particularly applicable for decarbonization in production of high chromium steel. Another technology has already proposed in Japanese Patent First Publication (Tokkai) Showa 47-17619, in which a solid phase oxygen is added as decarbonization promoting agent. Furthermore, Japanese Patent First Publication (Tokkai) Showa 55-125220 discloses decarbonization technology of top blowing by means of lance with Laval nozzle. These prior proposed technologies are effective for promoting decarbonization. However, non of these can solve a problem of temperature drop of the melt caused during degassing and/or decarbonization process.
It is the conventional practice to provisionally heating melt at a temperature higher than necessary temperature so as to compensate temperature drop during decarbonization process. In such conventional approach, a problem is encountered by higher temperature of the melt. Namely, when the temperature of the melt is to be risen about the necessary temperature in a primary furnace, such as a converter, refractory on a smelting furnace and ladle will be subject substantial heat to cause melting.
On the other hand, Iron and Steel No. 11, Vol. 64(1978)S 635 shows RH-OB process for heating the melt during vacuum degassing process. Also, Japanese Patent First Publications 53-81416 and 59-89708 disclose a technologies of heating the melt by adding heat generating material, such as Al, Si and so forth to the melt in the vacuum chamber or in the ladle and injecting oxygen gas to the heat generating material containing melt.
The following is one of typical process for actually implementing degassing and decarbonization process to utilize the foregoing methods set forth above in combination. In such actually implemented process,
(1) a molten steel for which deoxidation process is not performed, is subject decarbonization process for removing or reducing carbon content in the molten steel, thereafter, heat generating material, such as Alm Si or so forth is added to the molten metal for heating the molten steel by supplying oxygen
(2) the molten steel is heated by adding the heat generating material and by supplying oxygen, Al and Si in the molten steel is completely burned, and after completely burning the heat generating material, decarbonization process is performed
(3) as proposed in Japanese Patent First Publication (Tokkai) Showa 55-125220, in case of high chromium steel, chromium is oxidized by supplying oxygen for generating a heat for heating the molten steel.
In case of the solution (1) and (2) set forth above, since processing period is separated between a period for performing decarbonization and a period for heating the molten steel, process period is substantially expanded to degrade production efficiency. Particularly, in cause of the solution (1), substantial process time is required for completely combustioning Al and Si in case of high carbon steel. Similarly, in case of solution (2), substantial process period is required simply for completely combustioning Al and Si. Furthermore as a result of combustion, soniums, i.e. Al.sub.2 O.sub.3 and SiO.sub.2, is produced in the montel steel to cause degradation of the quality of the steel to be produced. Furthermore, addition of such heat generating material clearly cause increasing of production cost.
On the other hand, in case of the solution (3), since the content of the steel, such as chromium is consumed for generating heat, yield is clearly lowered.